Samphire, harvested from June to September, is cooked by steaming or boiling and is traditionally served with melted butter, as an accompaniment to sea-food. It is said to be similar to spinach or asparagus in flavour.
Marsh samphire is a plant that grows in coastal marshlands, especially in Norfolk. It is also known as glasswort because it was used until the early 19th century as a source of soda ash for glassmaking.
Not to be confused with rock samphire which is not eaten, as far as I know.